An example of a power supply device for a vehicle has been disclosed in JP 2015-83404 A. The power supply device disclosed in JP 2015-83404 A has a first power supply that supplies electric power to onboard loads, a second power supply that supplies electric power to the onboard loads, a connection switching unit that connects and disconnects the onboard loads and the second power supply, and a control unit that controls the connection switching unit. When the amount of power remaining in the second power supply is not within a predetermined range, the connection switching unit is controlled to perform a first control operation in which the second power supply is disconnected from the onboard loads. When it has been determined based on the state of the first power supply that power has to be supplied to the onboard loads from the second power supply, even if the amount of power remaining in the second power supply is not within the predetermined range, a second control operation is performed, which is given priority over the first control operation, in which the second power supply remains connected at least to a specific onboard load among the onboard loads.